Line6 is a manufacturer of a guitar amp.

http://uk.line6.com/

Quote from: Enzo on June 30, 2015, 08:49:43 PM
Any control that affects the hum is AFTER the source of the hum.  So whatever stage is offending, it is before the EQ.  Since you report the gain controls has an effect, that tends to localize the problem to the first stage - the three transistors just to the right of the input jacks.  So do look at the voltages.

I do agree however if you look at the fuzz isolation and clipper circuit, there is a path through that section which has not been ruled out, as it is all before the gain control.

 The screen grid resistors need lowering from 1K to 470R.
6550 has a higher grid leakage current than EL34

6550 needs more negative bias than the EL34 therefore the typical higher values for the resistors should be lowered to around 47K.

Only reading from old notes on Marshal mods so bias resistor may not be quite correct value.

Its OK I was thinking of another Peavey with a base board, ie the main board screwed to the base of the head unit.

Anyhow, back to the amp, as the noise goes when you reduce the gain, then you can rule out the intermediate stage and after that, which includes the EQ.

Arn't cct diagrams wonderful, when drawn by idiots.

There are a number of decoupling capacitors on the rails which could effect hum, however as has been said before, the jacks can be the main culprit.

At this point and after you have checked the jack sockets, you could do the following.

A simple check is to get a 100nF to 1uf cap and attach one end to earth, then use the other end to probe the signal path. This decouples the signal to ground and should helps you pinpoint where the fault is, without having to desolder any components. Don't do this on Valve amps

If you look at the circuit you can see that the signal goes through a small filter gain stage before heading into the effects stage via a 10K resistor. You could just remove one end of the 10K or decouple the signal.
If the hum goes, when the volume is left up then you have ruled out the effects side and shown that the problem lies within the first section of 3 transistors.

Quote from: g1 on June 30, 2015, 11:51:38 AM

Quote from: LateDev on June 30, 2015, 11:23:21 AM
You should never use the bulb method as part of any test

That is just plain wrong.
you're gonna be real disappointed around here  .

I guess I will just have to be disappointed and you will have to carry on in the mistaken belief it is a test device.

Quote from: ilyaa on June 30, 2015, 03:28:04 PM
unhooked the PT from the RC filter - and no tubes in the amp:

transformer side of the main filter cap shows short to ground.....boooo

its the can cap, for sure. all the other leads of the can (its 20/20 and 40/40) seem fine. would i be okay to just interject a separate large filter cap where that first section of the can cap is supposed to go?? think im gonna try that - seems a shame to replace the whole can if only of the sections is shorted, unless that means its time to toss it.....

That is a relief then. Am I to understand this is a single filter unit, or is it a double cap in a single can. Either way you should replace the entire filter, getting 2 caps and a resistor.
I would opt for larger caps anyhow as the DC side of the PSU is prone to droop at certain times which in itself causes current limiting and poor sound quality.

I must admit, people do get hung up on terminology.

An amplifier normally consists of two parts, the pre-amplifier which usually boosts the signal up to line level, and the power amp which takes this higher Voltage and increases it to whatever the amp is designed for.

Line level can be different depending on when the amp was built and for what purpose.

Whenever someone speaks of a front end to an amp, they can either mean the front of the power amp, or the front of the preamp. It all depends on their point of reference.

The coil wire has an elliptical cross section.
This of course means that the wire can still have the same current carrying capacity as a round wire with the same total cross sectional area, and the same number of windings as a round wire will fit in a smaller gap, or you can increase the number of windings for the same gap.

Speaker efficiency is increased by increasing the maximum amount of magnetic flux.

There are other factors to consider with Celestions from the older speaker range, as in they had a cambric edge surround which stiffened the movement and some manufacturers preferred a metal dome, rather than a fabric dome over the centre pole. I can't remember but I think Fender may have had the metal dome.

Quote from: Enzo on June 29, 2015, 11:55:55 PM
I don't see where he said he had no heater voltage.

If the power transformer had melted in any way, then it ought not matter if the rectifier was installed or not.  Shorted turns would draw current and light his bulb limiter with or without the rectifier.

True, I must have misread the heater part, and merely took it that there was no heater, as he had said there were no Voltages around the circuit.

A bulb limiter is a very rough way of determining a fault and the bulb can either light or not light depending on many factors, including the wattage of the bulb. You should never use the bulb method as part of any test, its use is to protect the amplifier under fault conditions, no more than that. Putting a bulb in line with a working amp is decidedly the wrong thing to do as well.

A transformer does not need all its windings melted to cause such a problem, however as you suggested there can be many other things which could cause this problem, even down to a piece of dislodged wire.

At least the area in which the problem lies has been narrowed down.

Have to agree with Enzo,

Here are the stats on the 2533, the page has an equivalent search on it
http://alltransistors.com/transistor.php?transistor=13340

Interesting and as far as warming on a tape is concerned, I tend to think that statement is both true and false at the same time.

I must admit I have not read it all as I did some research into this many years ago, so can see where the writer is coming from.

In the 80's there were always those that either swore by analogue or by digital. The analogue sound being truer to a live performance, hence warmer, and the digital notably different as having no warmth.

With this article the writer talks about different types of wave modification, the easiest is of course a straight clipping. Even then this will produce complex harmonics which can either add or subtract from a sound.

What is being said is where an instrument has distortion of any nature the waveform is complex and should not be thought of in terms of a fundamental sine wave. When you then mix this complex waveform with other complex waveforms, the harmonics of the accumulated waveforms adds warmth to the sound.

This happens in any analogue mix and the medium to which it is being recorded can either add or subtract from the overall effect.

The best signal for any tape player is where the signal is near saturation. basically this is because of the way a tape is recorded. You need a maximum flux density induced into the tape so that background noise is obliterated

I will go back to my earlier statement of the warmth coming from the tape being both true and false. Its false because the warmth starts earlier in the process and true because then it is added to, by recording on the tape.

An interesting article that deserves an in depth read


 

Ignores most of that as rhetoric. Whenever someone replies to any forum post, they are answering either the main topic or the post immediately in front of the one they are posting.
I think people who read this thread will be able to tell the difference.

Quote
Does everything posted here require a full page filled up with formulas?

Not at all, just a simple line with a small explanation, to help those who find this a little over their heads, would help clarify, otherwise statements like it will only give > .1W sounds condescending

Quote from: J M Fahey on June 30, 2015, 06:58:24 AM

Now LateDev is attempting to change the rules in the middle of a game by introducing the back/old stock card ... but his original suggestion was, and I hate having to quote his own words:

QuoteAs far as the transformers are concerned, if you got to Hammond or Sowter, or indeed any company that produces quality transformers for audio, they will be able to supply whatever you want.

Am I missing something?
[/quote]

Again the pointless dig. I have used Sowter in the past and they have provided a transformer to the common specifications that they have used millions of times before.
What you decided to state was that if you went to a transformer manufacturer and specified a bespoke/custom transformer then it would cost a lot, which is not untrue, however a Valve output transformer with the parameters you specified is not bespoke, and the link I gave everyone, shows this to be true.

I am an Engineer with over 40 years experience and merely wished to help or possibly educate, so I will bow out here as it serves no purpose.

A lot of amps used Celestion Speakers and those speakers were common to Marshall as well. I know there was a round wire and an elliptical wired speaker done at that time. The elliptical wired speaker was more efficient given that the air gap could be smaller for the same number of turns. I would presume that would have been perfected now with a flatter wire used on high efficiency speakers pro speakers of all types.

lol I have 2 x 12" Celestions in the hi-fi. I did have RCF speakers in there, but they were way too good for hi-fi 

Congratulations on getting it sorted, there is no worse problem than noise on any amp and is one of the harder things to successfully track by remote.

I must have time slipped there, which is easy these days.

I saw Enzo had said for you to check all the Voltages and compare between the channels, which is good advice as Voltages can vary from the ideal on the circuit diagram.

The first thing you do with any repair job is to make sure that there is a supply and the Voltages around the circuit are correct, or at least within 10%. Having 2 channels allows you to compare these Voltages with relative ease.
No PSU on to any part of the circuit, no output.

Once you have done this, then you can start on signal tracing.

As far as the pre-amp is concerned you could of course modify the input or you can just bypass the whole front section up to point 14 and 35, but more about that later if you decide to go down that route.

The main thing is to get it working first

As you have stated that turning the effect volume up increases the hum I really need to see a good cct diagram of that section in particular. However I do believe that the level is at the beginning of the effects section so it shows that the hum is coming in from the pre-amp section.

I can't see that circuit too well, but if there is a jack input to the effects side then as Enzo has stated, this could be a cause. It could be as simple as Oxidisation of the contacts, which cleaning will sort.

Peavy put all their controls on a front panel which can lead to all sorts of noise problems. You need to make sure that all the leads going from the front panel to the base board are properly connected, paying particular attention to any leads that look like they are local to the effects section.

If you can get a higher resolution circuit diagram, or give a link to one,  that would be great.

I was merely giving added data, so find your comment quite extraordinary.
I also supply everyone with data and a circuit to show that a typical amp gives 40mW.

Quote from: J M Fahey on June 30, 2015, 01:17:42 AM
I hate saying "I told you so""  but can't avoid it:

:duh
How is showing people you are correct via a good example, worthy of that comment.

You could at least show that your assumption of less than 100mW was justified.

I find it quite strange that some people come out with blind statements and don't give out information to others, to help further their knowledge.

People do not learn anything if you expect them to rely on blind faith.

As far as the transformers are concerned, a lot of manufacturers do keep back stock of audio transformers as they are still extensively used, and I think you will find that the cost is not in the order you want to assume it is.
Do a search on the net http://comingsoon.radioshack.com/radioshack-audio-output-transformer/2731380.html#.VZJdvkZTsQo